1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Related Art
In general, a semiconductor device comprises a silicon substrate which has a plurality of semiconductor regions. The semiconductor device is identified as an IGBT (Insulated Gate Bipolar Transistor), a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or a Diode in accordance with the configuration and the positional relationship of the plurality of semiconductor regions. The semiconductor device further comprises a pair of main electrodes, wherein one of the main electrodes (a surface electrode) is formed on a top surface of the silicon substrate and the other main electrode (a reverse electrode) is formed on a bottom surface of the silicon substrate. In order to solder the semiconductor device to a ceramic substrate, the reverse electrode may comprise a plurarity of conductor layers.
Japanese Laid-Open Patent Publication H10-163467 discloses a semiconductor device comprising a silicon substrate and a collector electrode (one example of a reverse electrode), wherein the collector electrode comprises a plurarity of conductor layers. The collector electrode comprises a first conductor layer, a second conductor layer, and a third conductor layer. The first conductor layer makes contact with the surface of the silicon substrate and includes aluminum. The second conductor layer makes contact with the first conductor layer and includes titanium. The third semiconductor layer is separated from the first conductor layer by the second conductor layer and includes nickel. The first conductor layer can improve the electrical contact properties between the surface of the silicon substrate and the collector electrode. The second conductor layer can improve the adhesion properties between the first conductor layer and the third conductor layer, and can also prevent the nickel in the third conductor layer from diffusing into the first conductor layer. The third conductor layer can improve the adhesion properties between the collector electrode and the solder.
When this type of semiconductor device is soldered to the ceramic substrate, the semiconductor device is exposed to heat as the solder is fused to the collector electrode. In general, the amount of the heat evolved by fusing the solder is adjusted to a temperature between 200 and 450 degrees C. such that the heat does not influence the surface structure (such as the passivation mask or the surface electrode made of aluminum) of the semiconductor device. However, even if the temperature is adjusted to between such a range, a portion of the silicon existing in the silicon substrate will diffuse into the first conductor layer. In this case, a lot of voids are formed in the surface of the silicon substrate, while aluminum in the first conductor layer penetrates into voids. As a result, a lot of aluminum spikes are formed at the boundary between the silicon substrate and the first conductor layer.